Analysis of the intervalence absorption band edge in the mixed-valency Sb^III,V salt Rb_2.67SbCl₆

The variation in the shape of the long-wavelength tail of the intervalence charge-transfer band has been measured in single crystals of the class II Sb^III,V mixed-valency compound of Rb_2.67SbCl₆ from 6 to 310 K. Far-infrared and Raman spectra (50-1000 cm^-1) are also reported. The absorption constant is an exponential function of photon energy, i.e. the band edge obeys the Urbach rule. To explain the temperature dependence of the steepness of the exponential band edge it is necessary to include quadratically coupled lattice modes in addition to the linearly coupled intramolecular modes. The mean frequency of this lattice mode is 88(2) cm^-1, in the same range as the transverse optic mode (76 cm^-1).